Magnetic memory device

ABSTRACT

A memory cell ( 310 ) for a magnetic memory device ( 300 ) includes a free layer ( 311 ), a cap layer, an antiferromagnetic layer, and a synthetic antiferromagnetic layer which comprises two or more than two ferromagnetic layers that are antiferromagnetically coupled through non-magnetic space layers. The synthetic antiferromagnetic layer is pinned by antiferromagnetic layer. The antiferromagnetic layer and the synthetic antiferromagnetic layer form a synthetic antiferromagnetic pinned (SAFP) recording layer. The magnetization of the SAFP recording layer can be changed by combining a heating process and an external field induced from currents flowing along the bit line ( 320 ) and the word line ( 330 ). Therefore, a MRAM with high density, high thermal stability, low power dissipation and high heat tolerance can be achieved after introducing the SAFP recording layer due to the high volume and anisotropy energy of the SAFP recording layer.

FIELD OF THE INVENTION

The present invention relates to a magnetic memory device. Inparticular, it relates to a magnetoresistive random access memory devicewith a synthetic antiferromagnetic pinned multi-layer that writes databy thermal assisted techniques.

BACKGROUND OF THE INVENTION

Magnetoresistive random access memory (MRAM) devices are solid state,non-volatile memory devices which make use of the giant magnetoresistiveeffect. A conventional MRAM device includes a column of first electricalwires, referred to as word lines, and a row of second electrical wires,referred to as bit lines. An array of magnetic memory cells, located atthe junctions of the word lines and bit lines, is used to record datasignals.

A typical magnetic memory cell comprises a hard magnetic layer, a softmagnetic layer, and a non-magnetic layer sandwiched between the hardmagnetic layer and the soft magnetic layer. The hard magnetic layer hasa magnetization vector fixed in one direction. The orientation of themagnetization vector does not change under a magnetic field appliedthereon. The soft magnetic layer has an alterable magnetization vectorunder a magnetic field applied thereon, that either points to the samedirection, hereinafter “parallel alignment”, or to the oppositedirection, hereinafter “antiparallel alignment”, of the magnetizationvector of the hard magnetic layer. Since the resistances of the magneticmemory cell in the “parallel alignment” status and the “antiparallelalignment” status are different, the two types of alignment status canbe used to record the two logical states—the “0”s or “1”s of a data bit.

In a writing operation, an electric current passes through the word lineand the bit line adjacent to the memory cell. When the electric currentreaches a certain threshold, a magnetic field generated by the electriccurrent will switch the orientation of the magnetization vector of thesoft magnetic layer. As a result, the magnetization vector of the hardmagnetic layer and the soft magnetic layer will be changed from one typeof alignment, e.g. “parallel alignment”, to the other type of alignment,e.g. “antiparallel alignment”, so that a data signal in form of one databit can be recorded in the memory cell.

In MRAM structure design, a lower writing power dissipation and a highercell density are most desired. Unfortunately, a reduction of cell size,i.e. an increase in cell density, will lead to a reduction in theavailable energy (K_(u)V) to store the bit message. Further, the errorrate increases very rapidly as the cell size scales down. However, inorder to reduce the error rate, high anisotropy energy is required toovercome thermal fluctuations. Hard magnetic material has higheranisotropy energy compared with soft magnetic material, but in that casea higher writing current is required. The higher anisotropy energyresults in higher writing current density, which unfortunately has thedisadvantage of electro-migration.

In order to reduce the writing current for a high coercitivity MRAM,thermally assisted MRAMs are disclosed in U.S. Pat. No. 6,385,082, U.S.patent application 20020089874, JP patent application 2002208680, and JPpatent application 2002208681. Un-pinned ferromagnetic materials, inwhich the coercitivity decreases sharply as temperature increases, areused for the recording layer in the MRAMs disclosed therein.

In order to increase the thermal stability of the magnetic memory cell,recently, a Curie point written MRAM has been proposed to improve thestability of MRAM, as described in U.S. Pat. No. 6,535,416, and in apaper by R. S. Beech et al.: “Curie point written magnetoresistivememory” in J. Appl. Phys. 87, No. 9, pp. 6403-6405, 2000. In the Curiepoint written MRAM structure, a single pinned layer is used as storagelayer. The pinned layer has a higher anisotropy than an unpinned layer.The use of the pinned layer for information storage provides improvedthermal stability, allowing the cell size to be reduced before thermalinstability becomes a limiting factor.

In order to increase the MRAM cell density, the MRAM structure can besimplified as mentioned in U.S. Pat. No. 6,597,618, U.S. Pat. No.6,341,084, U.S. Pat. No. 6,317,375 and U.S. Pat. No. 6,259,644.

In analogy to a conventional single-layered magnetic media, thermalstability can be improved by introduction of antiferromagneticallycoupled magnetic layers as the magnetic memory cell size decreasesfurther (see, e.g., E. E. Fullerton et al.: “Antiferromagneticallycoupled magnetic media layers for thermally stable high-densityrecording” in Appl. Phys. Lett. 77, No. 23, p. 3806, 2000).

OBJECT OF THE INVENTION

Accordingly it is an object of the present invention to provide a MRAMwith high thermal stability, low power dissipation, and high heattolerance during thermal assistant writing.

SUMMARY OF THE INVENTION

In a first embodiment of the present invention, a high thermal stablemagnetoresistive random access memory (MRAM) unit is presented. The MRAMunit includes a substrate, a plurality of memory cells formed on thesubstrate, and a plurality of electrical wires electrically coupled tothe plurality of memory cells. Each of the plurality of memory cellsincludes a synthetic antiferromagnetic pinned (SAFP) recording layer,the SAFP recording layer itself including two or moreantiferromagnetically coupled ferromagnetic layers pinned by at leastone antiferromagnetic (AFM) layer. The plurality of memory cellstogether with the plurality of electrical wires are adapted to conduct aheating current therethrough, thereby heating a respective one of saidplurality of memory cells. Each of the plurality of electrical wires isadapted to conduct writing currents therethrough, the writing currentsoperable to change the magnetization of the SAFP recording layer for therespective heated memory cell.

In a second embodiment of the present invention, a method for writingdata in a MRAM unit is presented. In this embodiment, the MRAM unitincludes a plurality of memory cells, a bit line and a word line, thebit and word lines being in electrical contact with the plurality ofmemory cells. Each memory cell includes a SAFP recording layer, and eachSAFP recording layer includes two or more antiferromagnetically coupledferromagnetic layers pinned by at least one antiferromagnetic layer. Themethod includes the operation of raising the temperature of the SAFPrecording layer in an individual memory cell to approach or exceed itscritical temperature independently of other memory cells, thereby easilychanging the magnetization of the particular memory cell's SAFPrecording layer. The method further includes the operation of writing amagnetization state in the SAFP recording layer of the individual memorycell by passing a first current completely through the bit line and bypassing a second current completely through the word line.

In a third embodiment of the present invention, a method for performinga read operation in a MRAM unit is presented. In this embodiment, theMRAM unit includes a plurality of memory cells, a bit line and a wordline, the bit and word lines being in electrical contact with theplurality of memory cells. Each memory cell includes a SAFP recordinglayer and a free magnetic layer, and each SAFP recording layer includestwo or more antiferromagnetically coupled ferromagnetic layers pinned byat least one antiferromagnetic layer. The method includes the operationof applying currents through the bit line and the word line. The methodfurther includes the operation of determining the magnetization state ofthe SAFP recording layer, wherein the resistance states of the SAFPrecording layer are dependent on the relative angles between themagnetization vectors of said SAFP recording layer and said freemagnetic layer.

These and other features of the present invention will be betterunderstood when viewed in light of the following drawings and detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partially enlarged perspective view showing a conventionalCIP SV MRAM device;

FIG. 1B is an enlarged view showing a memory cell of the MRAM deviceshown in FIG. 1A;

FIG. 2A is a partially enlarged perspective view showing a conventionalCPP type device such as MTJ and CPP SV MRAM;

FIG. 2B is an enlarged view showing a memory cell of the MRAM deviceshown in FIG. 2A;

FIG. 3A is a partially enlarged perspective view showing a CIP SV MRAMdevice according to a first embodiment of the present invention;

FIG. 3B is an enlarged view showing a memory cell of the MRAM deviceshown in FIG. 3A;

FIG. 4A is a partially enlarged perspective view showing a CPP typedevice such as MTJ and CPP SV MRAM according to a second embodiment ofthe present invention;

FIG. 4B is an enlarged view showing a memory cell of the MRAM deviceshown in FIG. 4A;

FIG. 5A is a schematic view of a CIP SV MRAM structure heated by acurrent along a bit line through a memory cell in accordance with thefirst embodiment of the present invention;

FIG. 5B is a schematic view of the memory cell's writing by applying thecurrent along the bit lines and the word lines after the memory cell isheated in accordance with the first embodiment of the present invention;

FIG. 6A is a schematic view of a CIP SV MRAM structure heated by acurrent through a heat element under a memory cell in accordance with athird embodiment of the invention;

FIG. 6B is a schematic view of the memory cell's writing by applying thecurrent along the bit line and the word line after the memory cell isheated by the heat element of FIG. 6A;

FIG. 7A is a schematic view of a CPP MRAM structure heated by a currentthrough a CPP memory cell in accordance with the second embodiment ofthe invention;

FIG. 7B is a schematic view of the CPP memory cell's writing by applyingthe current along the bit line and the word line after the CPP memorycell is heated in accordance with the second embodiment of theinvention;

FIG. 8A is a schematic view of a CPP MRAM structure heated by a currentthrough a CPP memory cell and heat element in accordance with a fourthembodiment of the invention;

FIG. 8B is a schematic view of an equivalent circuit of the CPP MRAMstructure heated by the current through the CPP memory cell and a Zenerdiode of FIG. 8A;

FIG. 8C is an illustration of an I-V curve of a Zener diode;

FIG. 8D is a schematic view of the CPP memory cell's writing by applyingthe current along the bit lines and the word lines after the CPP memorycell is heated by the heat element and itself in accordance with thefourth embodiment of the invention; and

FIG. 9 is a view of the MR-H curves of a MRAM cell.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

It is pointed out that in the present application the term “criticaltemperature” is substantially the blocking temperature forantiferromagnetic (AFM) material, which can be less than the Néeltemperature. The critical temperature is substantially the Curietemperature for ferromagnetic material. The blocking temperature of anAFM layer is the temperature at or above which the AFM layer loses itsability to “pin” (i.e. fix) the magnetization of an adjacentferromagnetic layer below the Curie temperature of the adjacentferromagnetic layer.

As shown in FIGS. 1A and 1B, a conventional current-in-plane (CIP)spin-valve (SV) MRAM device 100 comprises a plurality of memory cells110, a row of bit lines 120 passing through the memory cells 110 and acolumn of word lines 130 passing underneath the memory cells 110. Anexemplary memory cell 110 as shown in FIG. 1B comprises a first magneticlayer 112 made of ferromagnetic material such as CoFe and/or NiFe, anon-magnetic layer 114 made of non-magnetic material such as Cu, asecond magnetic layer 119 made of ferromagnetic material such as CoFeand/or NiFe, and an antiferromagnetic (AFM) layer 118 made of AFMmaterial such as IrMn, FeMn and/or PtMn, etc. The above layers aredisposed in sequence as shown in FIG. 1B. The second magnetic layer 119has a fixed magnetization vector 169 pointing rightwardly, for example,which does not change its direction under an external magnetic field,hereinafter pinned layer. The AFM layer 118 serves to fix themagnetization of the second magnetic layer 119.

In a first case, the first magnetic layer 112 is configured to be a freelayer as a recording layer having its magnetization vector 162 alterableunder an external magnetic field. During a writing process, a word linecurrent 132 passing underneath the cell 110 and a bit line current 122passing through the memory cell 110 generate corresponding magneticfield changes the orientation of the magnetization vector 162 of thefirst magnetic layer 112, to point either to the right or to the left.When pointing to the right, the magnetization vector 162 becomesparallel to the magnetization vector 169 (of the second magnetic layer119), which represents a low magnetic resistance state of the memorycell 110. When pointing to the left, the magnetization vector 162 isantiparallel to the magnetization vector 169 (of the second magneticlayer 119), which represents a high magnetic resistance state of thememory cell 110. The “low” and “high” states correspond to binary databits “0” and “1” by which a data signal may be stored in the memorycell.

In a second case of thermal assistant writing, the first magnetic layer112 is configured to be a free layer serving as reading layer having itsmagnetization vector 162 alterable under an external magnetic field. Thesecond magnetic layer 119 is configured to be a pinned layer serving asrecording layer having its magnetization vector 162 alterable under anexternal magnetic field after heating. During a writing process, a bitline current 122 passes through the memory cell 110 to heat it, and aword line current 132 passes underneath the memory cell 110 to generatetogether with the bit line current 122 a corresponding magnetic fieldthat changes the orientation of the magnetization vector 169 of thesecond magnetic layer 119 to point either to the right or to the left.During a reading process, an alterable external magnetic field isapplied to the memory cell 110 changing the magnetization vector 162 ofthe first magnetic layer 112 to be the initial state oriented to theleft for the resistance detecting method or firstly oriented to the leftand then changing to the right for the resistance change detectingmethod. When the magnetization vector 169 points to the left, themagnetization vector 162 of the first magnetic layer 112 becomesparallel to the magnetization vector 169 of the second magnetic layer119, which represents a low magnetic resistance state of the memory cell110. When the magnetization vector 169 point to the right, themagnetization vector 162 of the first magnetic layer 112 is antiparallelto the magnetization vector 169 of the second magnetic layer 119, whichrepresents a high magnetic resistance state of the memory cell 110. The“low” and “high” magnetic resistance states correspond to binary databits “0” and “1” by which a data signal may be stored in the memory cell110.

FIGS. 2A and 2B show a conventional current-perpendicular-to-plane (CPP)MRAM device 200 which comprises a plurality of memory cells 210, a rowof bit lines 220 passing above the memory cells 210 and a column of wordlines 230 passing underneath the memory cells 210. An exemplary memorycell 210 as shown in FIG. 2B comprises a first magnetic layer 212 madeof ferromagnetic material such as CoFe and/or NiFe, a non-magnetic layer214 made of non-magnetic material such as Cu for CPP SV or insulatormaterial such as AIO for magnetic tunnel junction (MTJ), a secondmagnetic layer 219 made of ferromagnetic material such as CoFe and/orNiFe, and an AFM layer 218 made of AFM material such as IrMn, FeMnand/or PtMn, etc. The above layers are disposed in sequence as shown inFIG. 2B. The second magnetic layer 219 has a fixed magnetization vector269 pointing rightwardly. The AFM layer 218 serves to fix themagnetization of the second magnetic layer 219.

In a first case, the first magnetic layer 212 is configured to be a freelayer as a recording layer having its magnetization vector 262 alterableunder an external magnetic field. During a writing process, a word linecurrent 232 passing underneath the cell 110 and a bit line current 122passing through the memory cell 210 generate corresponding magneticfield changes the orientation of the magnetization vector 262 of thefirst magnetic layer 212, to point either to the right or to the left.When pointing to the right, the magnetization vector 262 becomesparallel to the magnetization vector 269 (of the second magnetic layer),which represents a low magnetic resistance state of the memory cell 210.When pointing to the left, the magnetization vector 262 is antiparallelto the magnetization vector 169 (of the second magnetic layer), whichrepresents a high magnetic resistance state of the memory cell 210. The“low” and “high” states correspond to binary data bits “0” and “1” bywhich a data signal may be stored in the memory cell.

In a second case of thermal assistant writing, the first magnetic layer212 is configured to be a free layer serving as reading layer and havingits magnetization vector 262 alterable under an external magnetic field.The second magnetic layer 219 is configured to be a pinned layer servingas recording layer having its magnetization vector 269 alterable underan external magnetic field after heating. During a writing process, aheating current passes through a selected memory cell 210 and heats it,and then a word line current 232 passes underneath the memory cell 210and a bit line current 222 passes above the memory cell 210 to generatea corresponding magnetic field that changes the orientation of themagnetization vector 269 of the second magnetic layer 219 to pointeither to the right or to the left. During a reading process, analterable external magnetic field is applied to the memory cell 210changing the magnetization vector 262 of the first magnetic layer 212 tobe the initial state oriented to the left for the resistance detectingmethod or firstly oriented to the left and then changing to the rightfor the resistance change detecting method. When the magnetizationvector 269 points to the left, the magnetization vector 262 of the firstmagnetic layer 212 becomes parallel to the magnetization vector 269 ofthe second magnetic layer 219, which represents a low magneticresistance state of the memory cell 210. When the magnetization vector269 points to the right, the magnetization vector 262 of the firstmagnetic layer 212 is antiparallel to the magnetization vector 269 ofthe second magnetic layer 219, which represents a high magneticresistance state of the memory cell 210. The “low” and “high” magneticresistance states correspond to binary data bits “0” and “1” by which adata signal may be stored in the memory cell 210.

FIGS. 3A and 3B show a current-in-plane (CIP) spin-valve (SV) MRAMdevice 300 for a magnetic memory device according to a first embodimentof the present invention, which CIP SV MRAM device 300 comprises aplurality of memory cells 310, a row of bit lines 320 passing throughthe memory cells 310 and a column of word lines 330 passing underneaththe memory cells 310. Each memory cell 310 according to the firstembodiment of the present invention and shown in FIG. 3B comprises atemplate layer 302, a free magnetic layer 311, a first non-magneticlayer 312, a first ferromagnetic layer 313, a first non-magnetic spacelayer 314, a second ferromagnetic layer 315, a second non-magnetic spacelayer 314 a, a third ferromagnetic layer 315 a, an antiferromagnetic(AFM) layer 316, and a cap layer 317. The above layers are disposed insequence as shown in FIG. 3B. Therefore, each of the first and secondnon-magnetic space layers 314, 314 a is sandwiched between twoneighboring ones of the first, second and third ferromagnetic layers313, 315, 315 a.

The free magnetic layer 311 serves as reading layer to detect themagnetization state of the first, second and third ferromagnetic layers313, 315, 315 a which serve together as recording layer to store data,and has its magnetization vector 362 alterable under an externalmagnetic field after heating. During a writing process, a bit linecurrent 322 passes through the memory cell 310 to heat it, then a wordline current 332 passes underneath the memory cell 310 and the bit linecurrent 322 passes further through the memory cell 310 to generate acorresponding magnetic field that changes the orientation of themagnetization vector 369 of the first ferromagnetic layer 313 to pointeither to the right or to the left. Due to the antiferromagneticcoupling of the first, second and third ferromagnetic layers 313, 315,315 a forming antiferromagnetic coupled ferromagnetic layers, theorientations of the magnetization vectors of the second and thirdferromagnetic layers 315, 315 a are adjusted depending on theorientation of the magnetization vector 369 of the first ferromagneticlayer 313. During a reading process, an alterable external magneticfield is applied to the memory cell 310 changing the magnetizationvector 362 of the free magnetic layer 311 to be the initial stateoriented to the left for the resistance detecting method or firstlyoriented to the left and then changing to the right for the resistancechange detecting method. When the magnetization vector 369 points to theleft, the magnetization vector 362 of the free magnetic layer 311becomes parallel to the magnetization vector 369 of the firstferromagnetic layer 313, which represents a low magnetic resistancestate of the memory cell 310. When the magnetization vector 369 pointsto the right, the magnetization vector 362 of the free magnetic layer311 is antiparallel to the magnetization vector 369 of the firstferromagnetic layer 313, which represents a high magnetic resistancestate of the memory cell 310. The “low” and “high” magnetic resistancestates correspond to binary data bits “0” and “1” by which a data signalmay be stored in the memory cell 310. The different resistance stateswhich are detectable via an appropriate electronic circuit (not shown inthe present application) represent the different magnetization states inthe MRAM device 300, and the different magnetization states in onememory cell 310 represent one data bit of a data signal stored in theMRAM device 300.

In this structure, the first, second and third ferromagnetic layers 313,315, and 315 a are pinned layers, which serve as recording layer andwhich are coupled antiferromagnetically through the first and secondspace layers 314 and 314 a. The free magnetic layer 311, and the first,second and third ferromagnetic layers 313, 315, and 315 a can bemagnetic material such as Ni, Fe, Co, or their alloys. The first andsecond space layers 314 and 314 a can be Ru, Rh, Cr, C, B, Ta, etc. ortheir alloys. The first non-magnetic layer 312 can be non-magneticmaterial such as Cu, Au, etc. The magnetization vector of the thirdferromagnetic layer 315 a is fixed through exchange coupling with theAFM layer 316. The AFM layer 316 can be made of AFM material such asIrMn, FeMn and/or PtMn, etc. Alternatively to the AFM layer 316, also ahard magnetic layer made of high coercitivity magnetic material such asTbCo, DyCo, TbDyCo, CrCoPt, etc. can be used. The AFM layer 316 and theantiferromagnetically coupled first, second and third ferromagneticlayers 313, 315, 315 a form a synthetic antiferromagnetic pinned (SAFP)multi-layer to be used as SAFP recording layer. When the temperature ofthe memory cell 310 has reached a value which approaches or exceeds thecritical temperature of the AFM material, an external field induced fromcurrents passing through the bit line 320 and the word line 330,respectively, can change the pinned magnetization vector direction ofeach of the first, second and third ferromagnetic layers 313, 315, and315 a. The critical temperature of the AFM material depends on thethickness of the AFM layer 316 and lies in the range of 100° C. and 230°C. for the AFM material IrMn. The pinned magnetization vector directionscan be detected via the resistance or a resistance change. As the first,second and third ferromagnetic layers 313, 315, and 315 a are coupledantiferromagnetically, a large pinning field or a large anisotropyenergy and a high storage volume can be achieved in this structure.Thus, a high density MRAM with high thermal stability can be realized.Furthermore, more pinned layers, which are coupled antiferromagneticallywith the mentioned SAFP multi-layer, can be inserted to increase therecording density. As the temperature increases, there is diffusion atthe interface between the AFM layer 316 and the third ferromagneticlayer 315 a. The diffusion may affect the fatigue properties of theMRAM. However, the first and second space layers 314 and 314 a canresist the diffusion and improve the fatigue properties of the MRAM.Further details on reducing the diffusion can be found in Y. K. Zheng etal.: “High thermal stability MRAM with SAF layer” in IEEE Trans. MAG.40, No. 4, pp. 2248-2250, 2004, the disclosure of which is incorporatedherein in its entirety. Also, the thin effective thickness of the SAFPmulti-layer benefited from the antiferromagnetically coupled first,second and third or more ferromagnetic layers can reduce the switchfield when the memory cell 310 size scales down. Thus, the powerdissipation can be reduced in the SAFP MRAM.

Since the first, second and third ferromagnetic layers 313, 315, 315 aserve together as SAFP recording layer, their individual magnetizationvectors can be represented by a common magnetization vector of therecording layer.

The template layer 302 is a seed layer enabling the deposition of thefree layer 311 with nearly ideal crystal structure. The cap layer 317 isa protecting layer, which can further improve the giant magnetoresistiveeffect of the memory cell 310.

FIGS. 4A and 4B show a current-perpendicular-to-plane (CPP) MRAM device400 for a magnetic memory device according to a second embodiment of thepresent invention, which CPP MRAM device 400 comprises a plurality ofmemory cells 410, a row of bit lines 420 passing above the memory cells410 and a column of word lines 430 passing underneath the memory cells410. Each memory cell 410 according to the second embodiment of thepresent invention comprises a template layer 402, a free magnetic layer411, a first non-magnetic layer 412, a first ferromagnetic layer 413, afirst non-magnetic space layer 414, a second ferromagnetic layer 415, asecond non-magnetic space layer 414 a, a third ferromagnetic layer 415a, an antiferromagnetic (AFM) layer 416, and a cap layer 417. The abovelayers are disposed in sequence as shown in FIG. 4B. Therefore, each ofthe first and second non-magnetic space layers 414, 414 a is sandwichedbetween two neighboring ones of the first, second and thirdferromagnetic layers 413, 415, 415 a.

The free magnetic layer 411 serves as reading layer to detect themagnetization state of the first, second and third ferromagnetic layers413, 415, 415 a, and has its magnetization vector 462 alterable under anexternal magnetic field, and the first ferromagnetic layer 413 isconfigured to be a pinned layer serving as recording layer having itsmagnetization vector 469 alterable under an external magnetic fieldafter heating. During a writing process, a heating current passesthrough a selected memory cell 410 and heats it, and then a word linecurrent 432 passes underneath the memory cell 410 and a bit line current422 passes above the memory cell 410 to generate a correspondingmagnetic field that changes the orientation of the magnetization vector469 of the first ferromagnetic layer 413 to point either to the right orto the left. Due to the antiferromagnetic coupling of the first, secondand third ferromagnetic layers 413, 415, 415 a, the orientations of themagnetization vectors of the second and third ferromagnetic layers 415,415 a are adjusted depending on the orientation of the magnetizationvector 469 of the first ferromagnetic layer 413. During a readingprocess, an alterable external magnetic field is applied to the memorycell 410 changing the magnetization vector 462 of the free magneticlayer 411. When pointing to the left, the magnetization vector 462 ofthe free magnetic layer 411 becomes parallel to the magnetization vector469 of the first ferromagnetic layer 413, which represents a lowmagnetic resistance state of the memory cell 410. When pointing to theright, the magnetization vector 462 of the free magnetic layer 411 isantiparallel to the magnetization vector 469 of the first ferromagneticlayer 413, which represents a high magnetic resistance state of thememory cell 410. The changes from “low” to “high” magnetic resistancestates and from “high” to “low” magnetic resistance states correspond tobinary data bits “0” and “1” by which a data signal may be stored in thememory cell 410.

In this structure, the first, second and third ferromagnetic layers 413,415, and 415 a are pinned layers, which serve as recording layer andwhich are coupled antiferromagnetically through the first and secondspace layers 414 and 414 a. The free magnetic layer 411, and the first,second and third ferromagnetic layers 413, 415, and 415 a can bemagnetic material such as Ni, Fe, Co, or their alloys. The first andsecond space layers 414 and 414 a can be Ru, Rh, Cr, C, B, Ta, etc. ortheir alloys. The first non-magnetic layer 412 can be a conducting layersuch as Cu, Au, etc. for CPP SV MRAM, or an insulator layer such asAlO_(x), ZnO_(x), TaO_(x), etc. for magnetic tunnel junction (MTJ) MRAM.The magnetization vector of the third ferromagnetic layer 415 a is fixedthrough exchange coupling with the AFM layer 416. The AFM layer 416 canbe made of AFM material such as IrMn, FeMn and/or PtMn, etc.Alternatively to the AFM layer 416, also a hard magnetic layer made ofhigh coercitivity magnetic material such as TbCo, DyCo, TbDyCo, CrCoPt,etc. can be used. When the temperature has reached a value whichapproaches or exceeds the critical temperature of the AFM material, anexternal field induced from currents passing through the bit line 420and the word line 430, respectively, can change the pinned magnetizationvector direction of each of the first, second and third ferromagneticlayers 413, 415, and 415 a. And the pinned magnetization vectordirection can be detected through the resistance or a resistance change.The first, second and third ferromagnetic layers 413, 415, and 415 awith the AFM layer 416 form a synthetic antiferromagnetic pinned (SAFP)multi-layer. High density, high thermal stability, low power dissipationand high heat tolerance can be achieved due to the high anisotropyenergy and large volume of the SAFP multi-layer. Further densityincrement can be achieved by inserting more pinned layers, which areantiferromagnetically coupled with the SAFP multi-layer.

Since the first, second and third ferromagnetic layers 413, 415, 415 aserve together as SAFP recording layer, their individual magnetizationvectors can be represented by a common magnetization vector of therecording layer.

The template layer 402 is a seed layer enabling the deposition of thefree layer 411 with nearly ideal crystal structure. The cap layer 417 isa protecting layer, which can further improve the giant magnetoresistiveeffect of the memory cell 410.

For further details of the memory cell 410 of the second embodiment ofthe present invention please refer to the analogous description of thememory cell 310 of the first embodiment of the present invention.

Referring now to FIGS. 5A and 5B, the writing operation of magnetizationstates in a CIP SV MRAM unit in accordance with the first embodiment ofthe present invention is shown.

In FIG. 5A there is shown a memory cell 501 which is already describedin detail in FIG. 3B as memory cell 310, a bit line 502 and a word line503. The memory cell 501 is heated by current 515 present along the bitline 502. In a typical MRAM unit, the memory cell 501 is formed on asubstrate (not shown) of the MRAM unit. The bit line 502 and the wordline 503, which are electrically conductive layers, are also formed onthe substrate.

Referring to FIG. 5A, in operation, the memory cell 501 is heated byapplying a current 515 through the memory cell 501 along the bit line502. When the temperature of the recording layer approaches or exceedsthe critical temperature (the critical temperature being the blockingtemperature of the AFM layer or being the Curie temperature of the hardmagnetic layer, respectively, in the specific embodiments), thecoercitivity of the AFM layer/hard magnetic layer will decrease to nearzero, and the AFM layer/hard magnetic layer will loose its function ofpinning the common magnetization vector of the recording layer. A smallmagnetic field induced by writing currents 516, 517 in the bit line 502and the word line 503, respectively, will change the direction of thecommon magnetization vector of the recording layer (compare descriptionof FIG. 5B). After the temperature of the heated memory cell 501 dropsto ambient temperature, the common magnetization vector of the recordinglayer will be maintained to the set direction. The memory cell 501 isstable with respect to the recorded magnetization due to the highanisotropy energy reached again after cooling down the memory cell 501to ambient temperature.

Referring now to FIG. 5B, there is shown a writing process to the memorycell 501. After the memory cell 501 is heated, the memory cell'sswitching field reduces as described with reference to FIG. 5A. Currents516 and 517, applied along the bit line 502 and the word line 503,respectively, will induce a magnetic field, causing the recordinglayer's common magnetization vector to be changed. As it would be knownto one of ordinary skill in the art, the degree of change of the commonmagnetization vector will depend on the amount of current applied andthe magnitude of the induced magnetic field. According to the invention,the combined application of a heating current heating the memory cell501 and of writing currents switching the common magnetization vector ofthe recording layer of the memory cell 501 results in a considerabledecrease in necessary writing current intensity. The recording layer isa SAFP layer and the reading layer is a free magnetic layer or a pinnedlayer made of soft magnetic material, which has a higher criticaltemperature than the SAFP recording layer. In the heat assisted writingto the MRAM unit, the memory cell can be written with small writingcurrents just after the heating process due to the reduced barrierenergy height in the heated SAFP recording layer.

In a third embodiment of the present invention, a heat element 618 isprovided under the memory cell 501 of the first embodiment of thepresent invention, as shown in FIG. 6A for a CIP SV MRAM structure.Therefore, a description of the already described components of thememory cell 501 of the first embodiment of the present invention isomitted. In order to heat the memory cell 501 independently, the heatelement 618 may be placed under or above the memory cell 501. When avoltage is applied between the bit line 502 and the word line 503, acurrent 619 will heat the heat element 618, which will in turn heat thememory cell 501.

In FIG. 6B the memory cell's writing operation is shown by applying acurrent 516 along the bit line 502 and a current 517 along the word line503 after the memory cell 501 is heated by the heat element 618. Thecurrents 516, 517 along the bit line 502 and the word line 503,respectively, induce a magnetic field, which is used to set the commonmagnetization vector of the recording layer of the memory cell 501. Whenmultiple memory cells 501 are formed into an array in a MRAM device, itis possible that the heat element 618 will also partially heat othermemory cells because of a shunting effect. As it would be known to a5person of ordinary skill in the art, in order to suppress the shuntingeffect, a diode,5 field-effect transistor (FET), CMOS transistor, orother non-linear element (NLE) can be integrated with the heat element618.

Referring now to FIGS. 7A and 7B, a CPP type MRAM structure according tothe second embodiment of the present invention is shown, similar to theCIP structure in FIGS. 6A and 6B. In FIG. 7A, the CPP (MTJ or CPP SV)MRAM structure comprises a CPP memory cell 723 which is alreadydescribed in detail in FIG. 4B as memory cell 410, a bit line 722 and aword line 724. An initial heating current 725 is applied to the CPPmemory cell 723 by the bit line 722 and the word line 724.

The writing operation of a CPP memory cell 723 in a MRAM is similar tothe writing operation of the CIP memory cell 501 described earlier withrespect to FIG. 5B. FIG. 7B illustrates the memory cell's writingoperation wherein currents 726, 727 are applied along the bit line 722and the word line 724, respectively, after the memory cell 723 is heatedby an initial heating current 725.

In a fourth embodiment of the present invention, a CPP MRAM is shown inFIG. 8A, wherein a heat element 828 is provided under the memory cell723 of the second embodiment of the present invention and wherein theCPP MRAM structure is heated by a current 829 flowing through the CPPmemory cell 723 and a heat element 828. The components of the memorycell 723 are analogous to those of previously described memory cell 410.The heat element 828 can be a non-linear element, such as a Zener diode,a FET or any other suitable non-linear element. An equivalent circuit ofa MTJ memory cell integrated with a Zener diode is illustrated in FIG.8B. Details about the usage of a Zener diode as non-linear heatingelement are described in the international patent applicationPCT/SG03/00045, the disclosure of which is incorporated herein in itsentirety.

As shown in FIG. 8C, an I-V curve of a Zener diode is illustrated. Themaximum heating power (P_(max)) is equal toV_(d)×V_(b)/R_(m)+V_(b)×V_(b)/R_(m) wherein V_(d) is the voltage appliedacross the Zener diode, V_(b) is the breakdown voltage of the memorycell, and R_(m) is the memory cell resistance. In the forward biasedstate, the low voltage drop across the Zener diode can be used to selecta particular memory cell during reading. The Zener diode can also serveas a memory cell selector while writing. The typical voltage drop isabout 0.7 V and the typical breakdown voltage is about 1 V for a MTJmemory cell. In operation, the power from these voltage drops may not besufficient to heat the recording layer. However, in the reverse biasedstate, the breakdown voltage of the Zener diode can be larger than 4 V.The large voltage drop in this instance can be used to heat the Zenerdiode, and thereby heat the recording layer. The other unselected Zenerdiodes in the MRAM device are biased below the breakdown voltage, sothere is no shunt current flowing through the other unselected memorycells and Zener diodes. Thus, the shunting effect even while heating thememory cell can also be suppressed sufficiently by introducing anon-linear element such as Zener diodes or other FETS and diodes.

Referring to FIG. 8D, a CPP memory cell's writing operation is shownsimilar to the writing operations described above with respect to FIG.7B, wherein currents 726, 727 are applied along the bit line 722 and theword line 724, respectively, after the memory cell 723 is heated by theheat element 828.

FIG. 9 shows an experimental result for magnetoresistance curves of amagnetic memory device according to the first embodiment of the presentinvention which was shown in FIG. 3B. The X-axis represents the externalfield H (Oe), and the Y-axis represents the magnetic resistance R (ohmor Ω).

These magnetoresistance curves demonstrate writing to a memory cell of aSAFP MRAM and reading from the memory cell according to the presentinvention. After heating the memory cell by means of a heating currentof 3 mA, and followed by applying a writing field of H_(w)=75 Oe, whichcan be generated by means of writing currents, the resistance measurableacross the memory cell 310 decreases as the external field increases(shown as circle line), which means the magnetization vector of thepinned first ferromagnetic layer 313 is oriented rightward. However,after heating the memory cell 310 by means of a heating current of 3 mA,and followed by applying a writing field of H_(w)=−75 Oe, the resistancemeasurable across the memory cell 310 increases as the external fieldincreases (shown as rectangular line), which means the magnetizationvector of the pinned first ferromagnetic layer 313 is oriented leftward.These magnetoresistance curves show that the orientation of themagnetization vector of the pinned first ferromagnetic layer 313 can beset by combining the heating and writing fields or currents, and themagnetization vector direction of 313 can be detected by changing themagnetization of the free layer 311.

Whilst the present invention has been described with reference topreferred embodiments it should be appreciated that modifications andimprovements may be made to the invention without departing from thescope of the invention as defined in the following claim.

1. A magnetoresistive random access memory (MRAM) unit comprising: asubstrate; a plurality of memory cells formed on said substrate; and aplurality of electrical wires electrically coupled to said plurality ofmemory cells; wherein each of said plurality of memory cells comprises asynthetic antiferromagnetic pinned (SAFP) recording layer, wherein saidSAFP recording layer comprises two or more antiferromagnetically coupledferromagnetic layers pinned by at least one antiferromagnetic layer, andwherein said plurality of memory cells together with said plurality ofelectrical wires are adapted for a heating current flowing therethroughto heat a respective one of said plurality of memory cells and whereinsaid plurality of electrical wires is adapted for writing currentsflowing therethrough to change a magnetization of said SAFP recordinglayer of said heated respective one of said plurality of memory cells.2. The MRAM unit according to claim 1, wherein between each twoneighboring ferromagnetic layers of said two or moreantiferromagnetically coupled ferromagnetic layers a non-magnetic spacelayer is sandwiched.
 3. The MRAM unit according to claim 1, wherein eachof said plurality of memory cells further comprises a free magneticlayer and a cap layer.
 4. The MRAM unit according to claim 1, furthercomprising a plurality of heat elements, each of said plurality of heatelements being thermally coupled, respectively, to each of saidplurality of memory cells.
 5. The MRAM unit according to claim 4,wherein each of said plurality of heat elements is adapted to heat saidrespective one of said plurality of memory cells to a value approachingor exceeding the critical temperature of said SAFP recording layer. 6.The MRAM unit according to claim 4, wherein each of said plurality ofheat elements is a non-linear element.
 7. The MRAM unit according toclaim 6, wherein said non-linear element is one of the group comprising:a Zener diode and a field-effect transistor.
 8. The MRAM unit accordingto claim 1, wherein each of said plurality of memory cells is acurrent-in-plane memory cell.
 9. The MRAM unit according to claim 8,wherein said current-in-plane (CIP) memory cells are CIP spin-valvememory cells.
 10. The MRAM unit according to claim 1, wherein each ofsaid plurality of memory cells is a current-perpendicular-to-plane (CPP)memory cell.
 11. The MRAM unit according to claim 10, wherein said CPPmemory cells are magnetic tunnel junction memory cells.
 12. The MRAMunit according to claim 10, wherein said CPP memory cells are CPPspin-valve memory cells.
 13. The MRAM unit according to claim 1, whereinsaid free magnetic layer comprises magnetic material with highercritical temperature than that of said SAFP recording layer.
 14. Amethod of writing data in a MRAM unit which comprises a plurality ofmemory cells, a bit line and a word line both in electrical contact withsaid plurality of memory cells, each of said plurality of memory cellscomprising a synthetic antiferromagnetic pinned (SAFP) recording layer,said SAFP recording layer comprising two or more antiferromagneticallycoupled ferromagnetic layers pinned by at least one antiferromagneticlayer, said method comprising: raising the temperature of said SAFPrecording layer in an individual memory cell to a value approaching orexceeding its critical temperature independently of other memory cells,thereby reducing the coercitivity of said SAFP recording layer; andwriting a magnetization state in said SAFP recording layer of saidindividual memory cell by passing a first current completely throughsaid bit line and by passing a second current completely through saidword line.
 15. The method according to claim 14, further comprisingcooling down said SAFP recording layer in said individual memory cell tonearly ambient temperature after writing said magnetization state insaid SAFP recording layer of said individual memory cell.
 16. The methodaccording to claim 14, wherein raising the temperature of said SAFPrecording layer in said individual memory cell to a value approaching orexceeding its critical temperature independently of other memory cellsfurther comprises passing a heating current partly through said bitline, completely through said individual memory cell and partly throughsaid word line.
 17. The method according to claim 16, said methodfurther comprising providing a heating element adjacent said individualmemory cell before said step of raising the temperature of said SAFPrecording layer in said individual memory cell to a value approaching orexceeding its critical temperature independently of other memory cells.18. The method according to claim 17, wherein passing a heating currentpartly through said bit line, completely through said individual memorycell and partly through said word line further comprises also passingsaid heating current completely through said heating element.
 19. Themethod according to claim 17, said method further comprising providing anon-linear element as heating element.
 20. The method according to claim19, said method further comprising providing a Zener diode or afield-effect transistor as non-linear element.
 21. A method ofperforming a read operation in a MRAM unit which comprises a pluralityof memory cells, a bit line and a word line both in electrical contactwith the plurality of memory cells, each of said plurality of memorycells comprising a synthetic antiferromagnetic pinned (SAFP) recordinglayer and a free magnetic layer, said SAFP recording layer comprisingtwo or more antiferromagnetically coupled ferromagnetic layers pinned byat least one antiferromagnetic layer, the method comprising: applyingcurrents through said bit line and said word line; and determining themagnetization state of said SAFP recording layer, wherein the resistancestates of said SAFP recording layer are dependent on the relative anglesbetween the magnetization vectors of said SAFP recording layer and saidfree magnetic layer.